Add documentation on the "Building MFEM" page for CMake install

src/building.md

@@ -239,3 +239,107 @@ git clone https://github.com/spack/spack.git
 cd spack
 ./bin/spack install -v mfem
 ```
+
+## Building MFEM with CMake
+To build a serial version of MFEM with CMake first create a build directory. For example, using a build directory named `build` inside the MFEM source directory:
+```sh 
+mkdir build 
+cd build 
+```
+
+Run the CMake configuration on the MFEM source directory.
+```sh 
+cmake ..
+```
+
+Run the build command associated with the CMake configuration, specifying the number of parallel build tasks with the `-j` flag (4 tasks in this case). 
+```sh 
+cmake --build . -j 4
+```
+### Parallel build using CMake
+To build a parallel version of MFEM with CMake first build METIS and Hypre as described above.
+From the MFEM source directory, create a build directory. For example, using a build directory named `build` inside the MFEM source directory:
+```sh 
+mkdir build 
+cd build 
+```
+
+Run the CMake configuration on the MFEM source directory using the `MFEM_USE_MPI` CMake variable to enable MPI. 
+This will automatically search for the system MPI implementation, the METIS installation (in `<mfem-source-dir>/../metis-4.0`), and Hypre installation (in `<mfem-source-dir/../hypre`).
+```sh 
+cmake .. -DMFEM_USE_MPI=YES
+```
+Alternatively, run the CMake configuration also using the `MFEM_FETCH_TPLS` CMake variable to enable fetching of Hypre and METIS.
+This will automatically download, configure, and build Hypre and METIS alongside MFEM (note that this option is **not** currently supported for GPU builds).
+```sh
+cmake .. -DMFEM_USE_MPI=YES -DMFEM_FETCH_TPLS=YES
+```
+For either CMake configuration approach, now run the build command associated with the configuration, specifying the number of parallel build tasks with the `-j` flag (4 tasks in this case). 
+```sh 
+cmake --build . -j 4
+```
+
+### Advanced configuration steps
+To build with CUDA:
+```sh 
+cmake .. -DMFEM_USE_CUDA=YES
+```
+To specify what CUDA architecture to target:
+```sh
+cmake .. -DCUDA_ARCH="sm_70"
+```
+The CUDA architecture is formatted as `sm_{CC}`, or just `{CC}`, where CC is the GPU compute capability of the target GPU without the decimal point. A list of NVIDIA GPU compute capabilities can be found in [the NVIDIA developers documentation](https://developer.nvidia.com/cuda-gpus). Multiple CUDA architectures can be targeted with a comma or semicolon separated list.
+```sh 
+cmake .. -DCUDA_ARCH="{ARCH1},{ARCH2},{ARCH3}"
+```
+or 
+```sh
+cmake .. -DCUDA_ARCH="{ARCH1};{ARCH2};{ARCH3}"
+```
+Other accepted architecture identifies are `"all"` which targets all CUDA architectures,
+`"all-major"` which targets all major versions `sm_{*0}`, and `"native"` which targets the visible GPUs on the system. 
+
+To build with METIS 5, after following the instructions to build METIS 5 above:
+```sh 
+cmake .. -DMFEM_USE_MPI=YES -DMFEM_USE_METIS_5=YES -DMETIS_DIR=../../metis-5.1.0
+```
+To build with HIP:
+```sh
+cmake .. -DMFEM_USE_HIP=YES
+```
+
+To specify what HIP architecture(s) to target:
+```sh
+cmake .. -DCMAKE_HIP_ARCHITECTURES="gfx942;gfx90a"
+```
+Multiple architectures can be targeted using a semi-colon separated list.
+The HIP architecture for different GPU models can be found in [the LLVM documentation](https://llvm.org/docs/AMDGPUUsage.html#processors)
+
+When building for GPUs, it is recommended to enable Umpire with `-DMFEM_USE_UMPIRE=ON`; if it is not automatically found by CMake the installation directory can be specified with `-DUMPIRE_DIR=<path-to-umpire-installation-dir>`. Umpire can be downloaded from 
+
+- [https://github.com/LLNL/Umpire/tags](https://github.com/LLNL/Umpire/tags)
+
+### Advanced build steps
+Different targets can be built with the --target flag in the build step 
+```sh 
+cmake --build . -j 4 --target <target-name>
+```
+To build the examples use the `examples` target (the executables will be in the `build/examples` directory).
+```sh
+cmake --build . -j 4 --target examples 
+```
+
+To quickly check if the code is successfully built using example 1/1p use the `check` target.
+```sh
+cmake --build . -j 4 --target check 
+```
+
+To build the miniapps use the `miniapps` target
+```sh
+cmake --build . -j 4 --target miniapps 
+```
+
+To build everything use the `exec` target
+```sh
+cmake --build . -j 4 --target exec 
+```